The present invention generally relates to a capacitive vacuum measuring cell.
It is known that pressures or pressure differences can be measured by applying pressure to a thin membrane and measuring its deflection. A known and suitable method for measuring the deflection is to design the membrane arrangement as a variable electrical capacitance where the capacitance change which correlates with the pressure change is evaluated by measurement electronics in the known manner. The capacitance is created by arranging a thin, flexible membrane very close to another surface and by depositing an electrically conductive film on both mutually opposed surfaces or by fabricating them from electrically conductive material. When pressure is applied to the membrane, the deflection changes the distance between the two electrodes which leads to an analyzable capacitance change of the arrangement. Sensors of this type are mass-produced from silicon. The flat basic body, as well as the membrane, often consist entirely of silicon. There are also versions that are made of composite materials such as silicon with a glass substrate. Such sensors can be produced very economically. However, in vacuum applications, pressure sensors of this type are normally usable only for higher pressures in the range of approx. 10−1 mbar to several bar. High resolution at pressures below 10−1 mbar is no longer achievable with silicon. One of the reasons for this is that the silicon surface reacts with the environment, which impairs the sensitive sensor characteristic. Already water vapor that forms part of normal atmospheric air leads to a corresponding reaction on the surfaces. The problem becomes even more serious when the sensor is used in chemically aggressive atmospheres. For this reason, attempts were made to protect such silicon sensors against external influences by passivating the surfaces. Attempts were also made to deposit protective coatings on the surfaces in order to improve the durability and the resistance against chemically aggressive environments as described in DE 41 36 987. Such measures are costly and, in the case of mechanically deformable parts such as membranes, have only limited success, in particular in highly aggressive media such as fluorine, bromic acid and their compounds which are typically used in vacuum etching processes.
For this reason, attempts were made to build vacuum measuring cells entirely from corrosion resistant materials such as Al2O3. A known arrangement of this type is shown in FIG. 1. The vacuum measuring cell consists of a ceramic plate (20) above which a membrane (22) is arranged with a small gap between the two of them and a fusible seal (21) between the ceramic plate (20) and the edge of the membrane. The ceramic plate (20) together with the membrane (22) forms a reference vacuum chamber (25) that is evacuated down during the manufacturing process through a pumping port and which is sealed with a seal (28). The mutually opposed surfaces of the ceramic plate (20) and the membrane (22) inside the reference vacuum chamber (25) are coated with electrically conductive material and connected to insulated external terminals on which the capacitance signal can be evaluated by means of an electronic device (not shown in the illustration). To achieve corrosion resistance, plate (20) and membrane (22) are both made of ceramic material such as Al2O3. This vacuum measuring cell in turn is arranged in a vacuum-tight housing (23) that features a port (24) which is connected to the media to be measured. Via port (24) of the vacuum measuring cell, the resulting measurement vacuum chamber (26) is sealed off against the membrane (22) by means of an elastomer seal (27) so that the pressures to be measured reach only the surface of the membrane (22). For the purpose of sealing, the entire cell is pressed via the ceramic plate (20) and membrane (22) against the elastomer seal (27). Up to now, vacuum measuring cells of this type have been usable only for higher pressures in the range of 0.1 mbar to 100 bar. In addition, this design leads to stress in the materials which, at lower pressures, for example<1 mbar, significantly impairs the reproducibility of measurement results and the resolution. The ceramic membranes (22) used so far have a thickness ranging from 279 μm to 2540 μm. Such designs are not suitable for achieving wide measurement ranges, in particular low pressures of 0.1 mbar to 10−6 mbar. In addition, designs of this type, as disclosed also in U.S. Pat. No. 5,553,502, are costly.
The objective of the present invention is to eliminate the disadvantage of the current state of the art. In particular the objective of the present invention is to implement an easy-to-produce, economical vacuum measuring cell that is suitable for measuring pressures from 10−6 mbar to 1000 mbar, in particular from 10−6 mbar to 1 mbar, with an accuracy of better than 1%, preferably better than 0.3% of the measured value. The measurement range can be covered or subdivided into several vacuum measuring cells or membrane versions according to the invention. In addition this vacuum measuring cell shall be corrosion resistant to aggressive media, have a compact design, and be economical to manufacture.